diff --git a/runtime/vm/compiler/backend/slot.cc b/runtime/vm/compiler/backend/slot.cc
index 91f9984d9a3..cde8d18a730 100644
--- a/runtime/vm/compiler/backend/slot.cc
+++ b/runtime/vm/compiler/backend/slot.cc
@@ -331,7 +331,10 @@ const Slot& Slot::GetCanonicalSlot(Thread* thread,
 
 FieldGuardState::FieldGuardState(const Field& field)
     : state_(GuardedCidBits::encode(field.guarded_cid()) |
-             IsNullableBit::encode(field.is_nullable())) {}
+             IsNullableBit::encode(field.is_nullable())) {
+  ASSERT(compiler::target::UntaggedObject::kClassIdTagSize <=
+         GuardedCidBits::bitsize());
+}
 
 const Slot& Slot::Get(const Field& field,
                       const ParsedFunction* parsed_function) {
diff --git a/runtime/vm/compiler/backend/slot.h b/runtime/vm/compiler/backend/slot.h
index cc5b6349841..06832c59900 100644
--- a/runtime/vm/compiler/backend/slot.h
+++ b/runtime/vm/compiler/backend/slot.h
@@ -241,7 +241,7 @@ class FieldGuardState {
   bool is_nullable() const { return IsNullableBit::decode(state_); }
 
  private:
-  using GuardedCidBits = BitField<int32_t, ClassIdTagType, 0, 16>;
+  using GuardedCidBits = BitField<int32_t, ClassIdTagType, 0, 20>;
   using IsNullableBit = BitField<int32_t, bool, GuardedCidBits::kNextBit, 1>;
 
   const int32_t state_;